The National Institute of Child Health and Human Development (NICHD) is conducting a study to determine whether urinary excretion of prostaglandin metabolites is altered in preeclamptic patients before the clinical onset of disease, and to determine the effect of oral calcium supplementation during pregnancy on the excretion of these metabolites. The Contractor shall measure metabolites of prostacyclin and thromboxane in approximately 700 urine specimens selected by the Project Officer from five clinical centers over the course of two years.